Variable-speed inverter drives are typically used to control motor output where a fixed speed drive is less efficient or not as desirable. Often, applications of variable speed drives have an inherently variable and/or intermittent duty.
In order to save component cost, components having a thermal time constant comparable with or greater than the duty cycle time of the application can be chosen for the drive and control circuitry so as to make use of their intentional short-time overload capability.
In order to avoid damage to a drive or control component by inadvertent overloading, a component's temperature can be monitored by a temperature measuring device, and the variable-speed drive can be arranged to shut-down if the temperature becomes excessive. When stopped, the component is able to cool down under no-load conditions.
Alternatively, component temperature can be estimated using a suitable thermal model by way of other measurements, for example but not limited to the current in the component together with the temperature in the proximity of the component.
In addition to component choice due to thermal profile, systems installed in the European Union are required to meet Electromagnetic Compatibility (EMC) limits as to the line current harmonics generated by, for example, an input rectifier of the inverter drive, as well as radio frequency interference (RFI) emission. This typically requires the installation of a substantial input choke in either the d.c. link or a.c. input circuit of the inverter drive, and an RFI filter in the a.c. input circuit, both of which carry the input current and have to be suitably rated. Therefore, the rating of the components chosen to help meet EMC criteria is of significant influence on circuit complexity, performance and cost.
As explained above in relation to inadvertent overloading of the components of the drive and control circuitry, component temperature monitoring is performed. The additional components positioned for EMC/RFI reasons may also be a cause of a system shut-down due to overloading (overheating). Therefore, additional monitoring of these components is often undertaken which adds to the cost and complexity of a circuit.
With increasing worldwide awareness and associated regulation of Electromagnetic Compatibility (EMC) and the effect of power line harmonics on neighbouring electronic equipment, it is likely that the requirement for a system to operate within defined EMC limits will spread to other parts of the world.
Therefore, there is a need to balance component choice of an underlying circuit as well as additional EMC/RFI mitigating components based both on thermal characteristics as well as suitability for their EMC/RFI performance.